Small electric motors, such as sub-fractional horsepower PMDC motors, also known as miniature motors, have many applications. They are popular in home and office equipment and are especially common in modern vehicles for operating a variety of accessories. Often, these applications are sensitive to EMI (electro-magnetic interference) and the motors must be shielded or suppressed to avoid generating excessive EMI.
Common suppression techniques involve filter circuits connected to or across terminals of the motor. Common filter circuits involve capacitors, inductors, varistors and combinations thereof.
For most applications a simple ring varistor fitted to the commutator and a line capacitor connected across the motor terminals is sufficient. For more strict regulations and/or larger motors line inductors and earth capacitors may be used in addition to or instead of the varistor and line capacitor.
However, while this works well for most motors we have discovered that some motors produce a higher EMI emission in the UKW frequency range (68-108 MHz range). One such motor is a 4 pole 6 slot PMDC brush type motor which is used in a vehicle air-conditioning system. Unfortunately, the traditional varistor and line capacitor filter does not suppress emissions in this frequency range as well as desired.